1. Field of the Invention
The invention relates to a method and to an apparatus for the non-volatile storage of the count of an electronic counting circuit in which the respective count is stored by electrically reprogramming a non-volatile data memory.
2. Description of the Prior Art
In the field of counting technology, especially in the automotive industry, mechanical counters are usually used, e.g. as odometers, in which the retention or storage of the respective count is unproblematical. With the development of non-volatile, electrically reprogrammable memories (EEPROMs) as in the known random access memories (RAMs), it has become possible to vary the stored information on one hand, while on the other hand it is also possible to store the changed information without the need for maintaining a supply voltage, as in the known read-only memories (ROMs). Thus, EEPROMs combine the desirable characteristics of both memory types, for which reason they can basically be applied wherever the changing data must be protected even after the supply voltage is shut off. They therefore are particularly well suited for the non-volatile storage of the count of an electronic counting circuit.
However, EEPROMs require a reprogramming time which ranges between 1 ms and 1 s in commercially available EEPROMs. This characteristic is an obstacle to reliable data protection in case of an unforeseen shutoff of the supply voltage, e.g. in case of a power-supply breakdown, because the stored information is undefined during the reprogramming period. If, in case of a breakdown, for instance, the supply voltage is shut off while reprogramming, the old as well as the new information may be lost in the memory area addressed.
This risk can be reduced by operating the EEPROM, together with the addressing circuits required, with a separate circuit fed by its own battery or by a suitable storage capacitor, making the non-volatile storage by and large independent of the extraneously supplied voltage. However, remaining disadvantages are not only an increased residual risk as to the dependability of the data protection, particularly after several brief supply voltage dips in short succession, but also the expense of its own supply circuit for the completion of a programming operation after a shut-off of the external supply voltage.
It is accordingly an object of the invention to provide a method and apparatus for the non-volatile storage of the count of an electronic counting circuit which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, and which allows safe storage of the count even when supply voltage dips occur, without requiring an additional circuit.